Check out these links to find out more about rope.

Rope Life Factors For Your Safety

Know you Knots or Hitches

Discard Point - when to retire your rope

Knots Can Reduce Rope Strength

Use Rope Properly

Storage of Rope

Knots Can Reduce Rope Strength

While it is true that a knot reduces rope strength, it is also true that a knot is a convenient way to attach a rope to tree limbs and other ropes. the strength loss is a result of the tight bends that occur in the knot. With some knots ropes can loose up to 50% of their strength which is part of the reason the work load limit should not exceed 10% of the rope strength.
Sheet Bend
The sheet bend is used instead of a square knot to tie two lines of different diameters together. Be sure to slide the smaller line down into the loop. The sheet bend creates approximately 50% rope strength reduction.
Bowline
The bowline will not slip or jam and is easily untied. It is generally used to attach the climbing line to saddle D-rings and to attach snaps or hooks to ropes. The bowline creates about 40% rope strength reduction.